1. Field of the Invention
The present invention relates to a wide-angle zoom lens system practicable for a small camera such as a video camera.
2. Description of the Prior Art
Recently, a tape deck is manufactured small in size, and the integrated rate of an electronic circuit has been improved. And a camera body of a video camera and such is manufactured extremely small in size and costs are remarkably low. However, concerning a photographing lens which is indispensable to a camera, its relative value to a camera body is increasing in weight, volume and cost every year, although its absolute value is being improved. Therefore, a photographing lens is very much required to be manufactured smaller in size and to be lowered in its cost.
In the view of a camera user, an improvement in specifics of a photographing lens, such as a wide angle, a high zooming ratio, and a large aperture, is required as well as in its size and cost. As shown in FIG. 1(A), standard specifics of a photographing lens for a video camera at present are: about 2.omega.=50.degree. of an angle of view at the wide-angle end (.omega. denotes a half angle of view); 6.times. in a zooming ratio; about 1.4-1.8 of an open f-number. In order to meet the above requirements of an improvement of a large-aperture lens and a high-zoom lens, a zoom lens whose open f-number is about 1.2 or whose zooming ratio is heightened to, for example, 8, 10, 12 or 16 by lengthening the focal length on the telephoto side is provided popularly.
However, to meet the above requirement of an improvement of a wide-angle lens, no zoom lens which includes an angle of view, of about 2.omega.=60.degree. (very much effective in indoor photographing and such), is generally proposed at all. Telephoto effect can be achieved in a camera body by picture image processing such as digital zooming, while wide-angle effect cannot be achieved by picture image processing in a camera body. Therefore, there is a strong request for early supply of a lens which meets the above requirement of an improvement of a wide-angle lens. The reason why no zoom lens capable of covering a wide angle is provided in spite of the request is described hereinafter.
It is comparatively easy to heighten a zooming ratio of a standard lens whose specifics are 2.omega.=50.degree. of an angle of view, 6.times. in a zooming ratio and 1.4-1.8 of an open f-number as described above, by lengthening the focal length on the telephoto side, because even when the focal length is slightly lengthened, the diameter or the weight of a front lens of a lens system is influenced little owing to its small angle of view at the telephoto side. Practically, a zoom lens whose angle of view at the wide-angle side end is 2.omega.=50.degree., whose zooming ratio is 8.times. and whose f-number is 1.7 is provided without being too big in size or increasing the number of lens elements, compared with a zoom lens whose zooming ratio is 6.times..
On the other hand, it is very much difficult to make the zooming ratio large by lengthening the focal length on the wide angle side. Its reasons are described as follows.
i) On the wide angle side, an angle of view is large. Therefore, the diameter of the front lens has to be considerably large so as to prevent a lack of illumination caused by vignetting of off-axial light rays at the front lens. When the diameter of the front lens is lengthened, the weight of the lens system is excessively increased, because the ratio of the front lens weight to the lens system weight is extremely high.
ii) On the wide angle side, the off-axial light rays are incident at a considerable angle. Consequently, off-axial aberration such as distortion and curvature of field is large, and the number of lens elements has to be increased a lot so as to correct the aberration.
Now, few lenses covering a wide angle about 2.omega.=60.degree. are proposed. In the U.S. Pat. No. 4,200,358, as shown in FIG. 1(B), a zoom lens whose angle of view is 2.omega.=56.degree., whose open f-number is 1.4 and whose zooming ratio is 9.5.times. is disclosed. In this case, though the angle of view is widened by only 6.degree. from 50.degree., the diameter of a front lens obviously becomes large and the number of lens elements of the front lens is increased. In the Laid-Open Patent Application No. Sho 62-153913, as shown in FIG. 1(C), a zoom lens whose angle of view is 2.omega.=83.degree. and whose zooming ratio is 8.times. is disclosed, although it is not for general use. For general use, an angle of view is not needed to be widened so much, but the degree of increase in number of front lens elements and in length of the diameter of the front lens show how difficult it is to make an angle of view wide. Thus, widening an angle of view more than 2.omega.=50.degree. is extremely difficult. So far, the problems of the large size and cost-up have not been solved. In a common video camera for general use, what is given priority is a request for a decrease in size and cost. As a result, such a wide-angle zoom lens as described above has never been used generally. In a field of a single lens reflex camera, a few wide-angle zoom lenses whose angle of view is about 2.omega.=60.degree. are proposed, but it is impossible to modify them so as to apply to a video camera.